Chromosomal passengers are proteins that are involved in coordinating the chromosomal and cytoskeletal events of mitosis. The passengers are present in cells as a complex with at least four members: Aurora B, a protein kinase; inner centromeric protein, an activation and targeting subunit; Survivin (function unknown) and Borealin (function also unknown). The kinase is activated at the onset of mitosis, at least partly accomplished by regulation of the levels of its constituents. As mitosis progresses, the kinase complex moves to a highly choreographed series of locations in the mitotic cell, activating key substrates at precise locations and specific times. Functions that require chromosomal passenger activity include chromatin modification (phosphorylation of histone H3), correction of kinetochore attachment errors, aspects of the spindle assembly checkpoint, assembly of a stable bipolar spindle and the completion of cytokinesis. The chromosomal passenger complex provides an essential mechanism for mitotic regulation.